Geology of the Pacific Northwest

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The Pacific Northwest of the United States and Canada is a geologically active region.

1 Active geologic faults
2 Active volcanoes
3 Ancient events
4 Recent events
- 4.1 Volcanoes
- 4.2 Earthquakes
5 Ongoing events
- 5.1 Volcanoes
- 5.2 Faults
6 Photo gallery
7 Wikipedia links
8 External links

Image of the Juan de Fuca Plate that produced the magnitude nine Cascadia Earthquake in 1700. USGS image

Mount St. Helens erupts on May 18, 1980

Active geologic faults, Seattle to Portland area

[edit] Active geologic faults

The Pacific Northwest's geology is dominated by the subduction of oceanic plates (the Gorda, Juan de Fuca, and Explorer) beneath the North American Plate in the Cascadia subduction zone. The subducted plates are also the source of the heat driving the volcanics of the Cascade Range.

[edit] Active volcanoes

Lava Butte, a cinder cone well east of the Cascades in the Central Oregon desert, erupted within the last 6000 years. Note lava flow on left.

Main article: Cascade Volcanic Belt

The Northwest has many volcanoes. Many of the active ones are part of the Cascade Range, though both active and inactive volcanoes lie to the east and west of the Cascades.

[edit] Ancient events

[edit] Floods

Between 13,000 and 11,000 BC, the Missoula Floods shaped much of the present-day Columbia River.

[edit] Landslides

The Bonneville Slide was a large landslide that blocked the Columbia Gorge close to modern-day Cascade Locks, Oregon forming a land bridge approximately 200 feet high. The river eventually removed it, but this event is remembered in local legends of the Native Americans as the Bridge of the Gods. The slide occurred sometime in the eighteenth century and may or may not have been tied to the 1700 Cascadia earthquake.

[edit] Volcanoes

The Oregon State Quarter honors the creation of Crater Lake after the volcanic eruption of Mount Mazama

Mount Mazama – once the tallest of the Cascades, and the site of present-day Crater Lake – erupted violently in about 5670 BC.

Mount Hood erupted in 1781-82, fumaroles on the summit still spew sulfurous gas.

Mount Meager erupted violently in about 2350 BC and sent an ash column approximately 20 km high into the stratosphere.

[edit] Earthquakes

The Juan de Fuca Plate produced the large 1700 Cascadia earthquake.

[edit] Recent events

[edit] Volcanoes

Mount St. Helens erupted on May 18, 1980 (see 1980 Mount St. Helens eruption).

Highway 302 after the Nisqually earthquake

[edit] Earthquakes

On February 28, 2001, the 6.8 Nisqually earthquake struck about 10 miles northeast of Olympia, close to Seattle, causing some structural damage and panic.

[edit] Ongoing events

[edit] Volcanoes

Mount Rainier from Tacoma. Rainier's still active

Mount Saint Helens, Washington, still active in 2004

Aside from Mount St. Helens, several volcanoes of the Cascades are still active, notably Mount Rainier, Mount Hood, and California's Lassen Peak.

[edit] Faults

Geologic evidence shows the Juan de Fuca Plate slips every 300-500 years, the most recent having been in 1700. The subduction zone lies just offshore, posing a serious earthquake and tsunami hazard. At present science cannot predict the next slippage.

[edit] Photo gallery

Abert Rim, Oregon, one of the highest fault scarps in the United States	Fort Rock, a tuff ring in Central Oregon	Mount Saint Helens, Washington, blows off steam.	Hole-in-the-Ground, a maar (explosion crater) in central Oregon
Beacon Rock, Washington, the core of an ancient volcano.	Mount Hood, Oregon, still active	Hells Canyon on the Oregon — Idaho border	Haystack Rock at Cannon Beach, Oregon, an old lava flow.
Basalt flow on Craters of the moon, Idaho	The Columbia Gorge, a rain forest carved by the Missoula Floods.	Oregon's Painted Hills.	Oregon Caves National Monument.
Yeager Rock, Washington, a glacial erratic	Palouse Falls, Washington, a remnant of the Missoula Floods	Dry Falls, Washington, a remnant of the Missoula Floods.	Hoodoos at John Day Fossil Beds, Oregon.
The Channeled scablands, Washington, a remnant of the Missoula Floods.	Mount Fee, Canada, an inactive volcanic plug.	The Sisters, a residual of the Missoula Floods in the Wallula Gap.	Mount Adams, Washington, still active
Steens Mountain, a fault-block mountain in Eastern Oregon.	The Three Sisters, Oregon. The South Sister has shown recent geologic activity.	Newberry caldera, Oregon	Belknap Crater, Oregon
Three Fingered Jack, Oregon, long dormant	Mount Shasta, California, dormant but likely to erupt again	Broken Top, a dormant Oregon stratovolcano.	Mount Cayley, Canada, seismically active
Lassen Peak, California, likely to erupt again	Moses Coulee, Washington, a remnant of the Missoula Floods.	Mount Garibaldi, Canada, seismically active.	The Willamette Meteorite, a glacial erratic.